This invention relates to mechanisms for rapidly dropping a neutron absorbing poison material into the core of a nuclear reactor, and in particular to mechanisms that are self-actuated when the reactor coolant temperature reaches a critical value.
Typical designs of the nuclear reactor in a liquid metal cooled system contemplate a multi-region core consisting of several different assembly types. Most assemblies contain fuel, blanket, or reflector material, which are arranged for the efficient generation of power and breeding of new fuel. Two other kinds of assemblies are distributed throughout the array of fuel and blanket assemblies: control assemblies for gradual control of the power level in the reactor, and safety assemblies for the rapid shut down (scram) of the reactor in the event of a major malfunction or other potentially dangerous condition.
Each type of assembly typically is hexagonal in shape and has a substantially integral perimeter. The fuel, blanket, and reflector assemblies typically contain their respective active materials in the form of an array of cylindrical rods that are held in fixed relationship with the assembly perimeter and with the reactor core generally. The control and safety assemblies contain neutron absorbing poison material that is movably disposed within the assembly and is typically held in a withdrawn position above the power producing portion of the reactor core until control or scram action is desired. Consequently, the control and safety assemblies extend above the core region in order to accommodate the axial motion of the poison material contained therein.
Most of the heat energy generated in the reactor core is produced in the fuel assemblies and is extracted from the core by means of a liquid metal coolant which enters at the bottom of every assembly, rises up through the assemblies, enters an upper plenum region and is directed away from the reactor core to a heat recovery system designed to produce electrical power. The most commonly used liquid metal reactor coolant material is sodium, which enters the reactor core at a temperature of about 340.degree. C., and leaves the core at a temperature of about 510.degree. C.
In the event of a malfunction or potentially dangerous condition, the power level is rapidly reduced by the rapid insertion of several poison safety (scram) rods, preferably by the use of a passive force such as gravity. In conventional nuclear reactor systems, instrumentation in the plant protective system (PPS) is relied upon to sense the malfunction and to produce an electrical signal to actuate a release mechanism that drops the safety rods into the reactor core. The instrumentation and release mechanism are typically external to the reactor core and the pressure vessel surrounding the core.
An additional level of safety, especially desired in the liquid metal fast breeder (LMFBR) type nuclear reactors, contemplates the use of self-actuated release mechanisms for the scram rods. These release mechanisms are directly actuated by a critical value of a system parameter such as low coolant flow rate, high power, or high core temperature, and do not rely on indirect sensors and instrumentation. It is desirable that such actuators and release mechanisms be entirely internal to the reactor vessel, thus isolating them from potentially damaging incidents such as explosions and missiles in the containment area surrounding the vessel. Self-actuated scram (SAS) mechanisms thus contain structure for sensing the system parameter, structure for actuating the safety rod release mechanism, the release mechanism, and the safety rod. The safety poison is typically in form of B.sub.4 C rods, but other absorbers such as tantalum spheres have also been proposed.
The reactor coolant temperature is one system parameter used to provide protection against a wide range of severe system malfunctions. Excessive coolant temperature results from a coolant flow rate that is too low for the desired power level, or a power level that is too high for the desired flow rate. Several prior art SAS mechanisms utilize the temperature-dependent phase change of a material located in fluid communication with the reactor coolant and designed as a "weak link" in holding the safety rod out of the reactor core during normal operation. When the reactor coolant reaches the melting point of the "weak link," the chain of support elements is broken and the safety rod falls into the reactor core. Another SAS mechanism utilizes the differential axial expansion of two metallic cylinders located in fluid communication with the reactor coolant to sense the coolant temperature rise and move a push rod, actuating the release of a mechanical gripper which holds up the safety rod.
Although the temperature sensitive SAS mechanisms in the prior art will scram the reactor at an excessive temperature, each is deficient in some important ways for use in large LMFBRs. These mechanisms either react too slowly to temperature changes or have a large uncertainty in the actuation temperature relative to the average reactor coolant outlet temperature. If the PPS mode of scram fails to operate, these deficiencies in the prior art SAS mechanisms will probably result in some damage to the fuel. Most prior art SAS mechanisms do not permit sharing of the safety rod release mechanism by the PPS and SAS modes of scram. Some prior art SAS mechanisms are not resettable after a scram, thereby necessitating different safety assemblies for PPS scram and SAS purposes.